1. Field of the Invention
The present invention relates to an apparatus for generating dehumidified air by employing an adsorbent such as of synthetic zeolite, and more particularly to a dehumidified air generator suitable for use in drying a plastic material which is to be molded to shape.
2. Description of the Related Art
Various known dehumidified air generators are generally grouped into (a) dehumidified air generators for use in chemical plants, (b) dehumidified air generators for use in air conditioning systems, and (c) dehumidified air generators for use in machines for drying plastic materials to be molded to shape. The dehumidified air generators under the category (c) are finding wide use as a source of dehumidified air for drying a plastic material (mostly in the form of pellets) down to a very low water content before it is charged into a plastic molding machine. These dehumidified air generating apparatus are of growing importance particularly because they are required for the manufacture of engineering plastics which have been developed in recent years.
Conditions for designing dehumidified air generators of the above classes (a) through (c) will be described below.
(a) Dehumidified air generators for use in chemical plants:
When a dehumidified air generator is designed for use in a chemical plant, it should produce air that has been dried to have a very low dew point (lower than -70.degree. C., for example). These dehumidified air generators are designed individually or customized in almost all applications. Since there are few apparatus size limitations, dehumidified air generators in this group can be designed to ideal specifications which meet operating principles of the generators. In many cases, large and complex dehumidified air generating systems are designed for desired performance.
(b) Dehumidified air generators for use in air conditioning systems:
Dehumidified air generators of this type find a larger market than those of the class (a). Dehumidified air produced by the dehumidified air generators is not required to have a considerably low dew point, but normally should have a dew point ranging from +10.degree. C. to +12.degree. C. (if air is to be dehumidified to a dew point of +12.degree. C., then air at 35.degree. C. may be dehumidified to a relative humidity of 5%). Therefore, the dehumidified air generators can easily be designed.
(c) Dehumidified air generators for drying plastic materials:
Depending on the type and use of plastics to be dried, air should be dehumidified by a dehumidified air generator in this category down to a dew point that ranges from -50.degree. C. to -60.degree. C. Therefore, the dew point requirement is similar to that of the class (a). Inasmuch as even relatively small-sized plastic molding machines require dehumidified air generators, respectively, cost and space limitations on these dehumidified air generators are strict. It is impossible to scale down a dehumidified air generator which was designed for use in a chemical plant and use it for drying a plastic material.
Heretofore, dehumidified air generators for use as plastic material driers have been manufactured and sold without regard for basic technical conditions that should be met when using an adsorbent. Naturally, such dehumidified air generators have proven unsatisfactory in use, and have posed problems especially on certain plastics such as PET, for example, which depend on their water contents introduced into the molding machine for determining the properties of products molded of those plastic materials. Where there are stringent requirements as to the dehumidification of plastics such as PET, most of the conventional dehumidified air generators cannot be used. Even if some of them can be used, they are oversized and require a wasteful expenditure of installing and running costs.
Technical conditions to be met by a dehumidified air generator which uses an adsorbent are as follows:
(1) Assuming that air flows in a normal direction for adsorption in each step, desorbing air at a high temperature ranging from 250.degree. C. to 300.degree. C. should flow in the direction opposite to the normal direction.
(2) Low-temperature air in a step after desorption, i.e., a cooling step, should flow in the normal direction.
(3) It is highly preferable that air to be used in the cooling step be dehumidified air.
(4) In an adsorbing step, there is an optimum air flow rate in a range determined by the type and particle diameter of an adsorbent. To keep the air flow rate in that range, it is necessary to provide a cross-sectional area of an adsorbent layer which is suitable for the flow rate of air to be dehumidified, and hold the cross-sectional area perpendicular to the direction in which the air to be dehumidified flows.
(5) It is necessary that the adsorbent layer have a minimum required thickness in the direction of flow of the air to be dehumidified, for achieving a desired dew point and keeping that dew point for a suitable period of time.
If a dehumidified air generator having a plurality of adsorbing columns satisfies the conditions (1) through (5) while successively switching between cycles of adsorbing and regenerating steps (i.e., desorbing and cooling steps), gas passages for the respective step cycles are complicated because of the conditions (1), (2), (3), and a number of automatic directional control valves are required to switch between the gas passages.
An adsorption drier including a flat-type slip valve which is free from the complex gas passages and the directional control valves is disclosed, for example, in Japanese Laid-Open Patent Publication No. 46-7440 claiming Convention priority based on German Patent Application No. P202505.5 filed on May 23, 1970. As shown in FIGS. 1 and 2 of this publication, a plurality of adsorption columns are horizontally arranged around a vertical rotational shaft, and discs are hermetically coupled to the upper and lower ends of the adsorption columns and positioned upwardly and downwardly of the adsorption columns. The disc is rotatable in unison with the rotors of valves mounted on the disc and held closely against a confronting nonrotatable disc. The nonrotatable disc has openings aligned respectively with the areas where adsorbing and regenerating steps are effected. The openings are held in communication through gas passages with the devices which perform the adsorbing and regenerating steps. In an actual system incorporating the principles of the disclosed adsorption drier, an adsorbent is stored in the gap between vertical double tubes to provide an adsorbent layer with a sufficient vertical cross-sectional area (as defined by the above condition (4)) with respect to air that flows in a horizontal direction. Since the adsorbent layer surface across and through which air passes in each of the adsorption columns extends vertically, it is possible to minimize the installation area for the adsorption drier.
If the adsorbent layer surface extended horizontally and air passed vertically therethrough in the disclosed arrangement, the diameter of each adsorbing column would be much greater than that which is disclosed in the publication. Consequently, while the disclosed adsorption drier can meet the conditions (1) and (2), it cannot satisfy both the conditions (4) and (5) while minimizing the installation space. More specifically, if the installation area is reduced, then it is unable to meet the condition (5) which requires the adsorbent layer to have a sufficient thickness. Another problem with the known structure in which air is horizontally passed across and through the vertical surface of the thin adsorbent bed is that the packing density of the adsorbent is rendered irregular due to gravity and hence the speed of air passing through the adsorbent layer is also made irregular. Moreover, the adsorption columns (normally four or five adsorption columns) which are disposed around the vertical rotatable shaft give a considerable large diameter to the entire rotatable assembly, and are not appropriate for use in a plastic molding plant as it takes up a large installation area.
Japanese Laid-Open Patent Publication No. 49-7166 claiming Convention priority based on German Patent Application No. P2214662.9 filed on Mar. 25, 1972, appears to disclose superficially a plurality of vertically stacked adsorption columns. However, it should be noted that actually, such vertically stacked adsorption columns are not disclosed in this publication for the following reason.
The external dimensions of directional control valves are shown as being quite large as compared with the diameters of inlet and outlet holes (1b, 11b). This indicates that FIGS. 2 and 4 of the publication are conceptual. Conversely, the dimensions of chambers (adsorption columns) filled with molecular sieves (adsorbent) in FIGS. 2, 4, and 6 of the publication are shown as much smaller than the external dimensions of the directional control valves. Technically, it is necessary that the external dimensions of the chambers be much larger than those of the directional control valves even if the directional control valves are not reduced in external dimensions. This fact also shows that FIGS. 2 and 4 of the publication are merely of a conceptual nature.
The publication fails to refer to the arrangement and advantages of the adsorption chambers, and does not specifically indicate how the adsorption chambers are attached to valve plates.
In addition, although FIG. 6 shows that the adsorption chambers are horizontally arranged and air flows horizontally, any comparison with a vertical stack of adsorption chambers is not described at all.
FIGS. 2, 4, 5, and 6 of the publication show the upper and lower valve plates in perspective, whereas lower components such as a heater, an air blower, etc., are depicted as a circuit diagram. Accordingly, the dimensions and positions of the adsorption chambers 5, 6 and conduits connecting them to the valve plates have no significance as indicating directions in these figures. FIGS. 2, 4, 5, and 6 serve the purpose of indicating the angular positions of the holes in the valve plates and their function, with the remaining portions being added simply as system diagrams.
As a consequence, FIGS. 2 and 4 do not indicate that the adsorption chambers are vertically stacked, and any vertical stack of adsorption chambers is not disclosed inasmuch as no operation and advantages whatsoever are described with respect to a vertical stack of adsorption chambers.